The present invention relates to a transmission system using interleaving, and more particularly, to one wherein the interleaving interval adapts to channel conditions.
Communication systems frequently use error correcting codes to reduce the error rate. However, when the channel has a burst of errors due to fading, errors occur in adjacent bits, which the code may have difficulty correcting In order to overcome this problem, interleaving (time spreading) is used so that originally adjacent bits are non-adjacently transmitted in the channel. At the receiver the original bit order is restored (deinterleaved). Thus, when adjacent burst errors occur in the channel, they are non-adjacent when the original bit order is restored. This allows the error correcting code to efficiently operate. The interleaver time span or interval before transmitting originally adjacent bits is fixed and is usually between 3 and 10 times the duration of the expected mean time between the fades (decorrelation time) of a selected depth, e.g., at least 3 dB. Interleaving intervals longer than 10 times the channel decorrelation time do not result in appreciable error rate reduction. This interleaver time span usually is between 5 and 20 seconds for an SHF (super high frequency) signal propagating through the ionosphere. However, if rapid fading occurs, e.g., due to ionospheric scintillations, then the long interleaving interval will not accomplish appreciable error rate reduction compared to a shorter one. The only effect will be to unnecessarily extend message transmission time. For example, if the channel is a portion of a network, e.g., five nodes (4 links), and if there is a 10 second message transmission delay in each link, the time delay for conveying a message from the first to the last link will be 40 seconds. For many applications this time delay is not acceptable.